Recently, due to the further increase in the integration density of large scale integrated circuits (LSIs), charged particle beam exposure methods such as an electron beam exposure method is expected to replace the photolithography technique which was conventionally used to expose fine patterns. The charged particle beam exposure method has many advantageous features. First, the stable drawing of fine patterns and the alignment accuracy can be controlled in a satisfactory manner. Second, a satisfactory focal depth is obtainable, and it is possible to guarantee no defects in the drawn patterns because no mask is used, thereby improving the reliability of the LSI. Third, it is possible to considerably improve the production yield at fine pattern regions of the LSI.
Therefore, the charged particle beam exposure method is characterized in that rectangular shots of the charged particle beam are made on an object surface and fine patterns on the order of microns or less can be drawn on the object surface. However, the charged particle beam exposure method draws the patterns by connecting the rectangular shots, and the patterns are drawn by a single stroke of the charged particle beam. As a result, the number of shots per unit area increases considerably as the size of the patterns decreases, and the throughput becomes poor when drawing the fine patterns.
On the other hand, in many situations, the patterns to be exposed include repeating patterns. Hence, the so-called block exposure method was developed. This block exposure method uses a mask which includes a plurality of openings for exposing basic patterns in a single shot of the charged particle beam which has a rectangular cross section, where the basic pattern forms a unit of the repeating pattern within the patterns which are to be drawn. According to the block exposure method, the basic patterns are repeatedly exposed and connected, so that the desired patterns are exposed at a high speed.
The mask which is provided with the openings corresponding to the basic patterns are often referred to as a block mask or a stencil mask, and the basic patterns are arranged in blocks of the block mask. For example, the basic patterns have rectangular or triangular shapes, so that the cross section of the charged particle beam transmitted through the opening of the block mask is shaped into such shapes. Further, the cross section of the charged particle beam transmitted through the opening of the block mask can be varied by partially or totally irradiating the opening by the charged particle beam. When making the block mask, it is essential that the openings corresponding to the basic patterns have shapes which are not easily damaged.
For example, the block exposure method which uses the block mask is proposed in a Japanese Laid-Open Patent Application No. 52-119185. Furthermore, the exposure method which uses the block mask provided with openings corresponding to repeating patterns of a memory cell or the like and to general rectangular patterns is proposed in a Japanese Laid-Open Patent Application No. 62-260322, for example.
The block exposure method is particularly effective when exposing patterns in which a majority of the exposing area is made up of a repetition of basic patterns. Hence, even for semiconductor devices such as a 64 Mbit dynamic random access memories (DRAMs) and 256 Mbit DRAMs which require extremely fine patterns, it is possible to obtain a satisfactory throughput which would enable mass production of such semiconductor devices by employing the block exposure method.
In order to efficiently carry out the exposure employing the block exposure method, openings corresponding to a plurality of basic patterns are provided in the block mask, and the opening corresponding to an arbitrary one of the basic patterns is selectively used. The opening is selected by deflecting the charged particle beam by a deflector having a relatively large deflection range, and the charged particle beam is deflected by a deflector having a relatively small deflection range if the charged particle beam is to partially irradiate the selected opening.
The shapes of the openings corresponding to the basic patterns must be limited to a certain extent, because some shapes cannot be realized on the block mask and the block mask will easily be damaged at parts where extremely complicated shapes are formed. In other words, not all kinds of basic patterns can be realized by use of the block mask. Conventionally, the shapes of the basic patterns obtained from the basic pattern data (block pattern data) are checked by the human eye to determine whether or not each pattern can be realized on the block mask and whether or not to allow each pattern to be provided on the block mask. Accordingly, it was conventionally necessary for an experienced and skilled operator to make the above check.
FIG. 1 shows examples of basic patterns which are prohibited from being provided on the block mask. In FIG. 1, the basic patterns, that is, the openings in the block mask if provided thereon, are indicated by non-hatched portions while the parts of the block mask having no opening are indicated by hatchings.
FIG. 1 (A) shows a rectangular frame shaped basic pattern. In this case, an opening 111 is formed around a rectangular region 110. However, since the rectangular region 110 cannot connect to a part of the block mask, it is physically impossible to realize this basic pattern.
FIG. 1 (B) shows a sideways U-shaped basic pattern. In this case, an opening 113 is formed adjacent to three sides of a rectangular tongue region 112. According to this basic pattern, there is a relatively long head conduction path, and the basic pattern is easily damaged because the temperature at the rectangular tongue region 112 easily rises to a high temperature.
FIG. 1 (C) shows a sideways C-shaped basic pattern. In this case, an opening 115 is formed adjacent to three sides of a rectangular region 114, and the remaining side of the rectangular region 114 connects to the surrounding via a narrow connecting part 114a. According to this basic pattern, the connecting part 114a narrow and may not be able to sufficiently support the weight of the rectangular region 114.
FIG. 1 (D) shows a basic pattern which corresponds to the surrounding of an L-shape. In this case, openings 117a and 117b surround an L-shaped region 116. According to this basic pattern, the L-shaped region 116 is narrow and is weak from the point of view of the tension introduced at this part.
FIG. 1 (E) shows a basic pattern which is made up of two parallel bands. In this case, openings 119a and 119b sandwich a narrow band-shaped region 118. According to this basic pattern, the narrow band-shaped region 118 easily breaks.
FIG. 1 (F) shows a basic pattern having the shape shown. In this case, openings 121a through 121d surround a narrow intersecting region 120. According to this basic pattern, the narrow intersecting region 120 easily breaks.
Conventionally, the above described basic patterns shown in FIG. 1 which should be prohibited from being provided on the block mask are checked by the human eye. However, the conditions under which the basic patterns to be prohibited are judged by such a check cannot be indicated by numerical values because the check relies on the human eye, and the conventional check is totally dependent on the experience and skills of the operator who makes the check.
Therefore, there was a problem in that the basic patterns which may or may not be provided on the block mask cannot be judged objectively and accurately for all kinds of basic patterns. As a result, the basic patterns provided on the block mask may easily be damaged and broken due to heat and insufficient support, and on the other hand, the basic patterns which will not generate problems and may actually be provided on the block mask may be prohibited from being provided on the block mask as a result of the check which relies on the human eye.